An avalanche photodiode (APD) is a semiconductor device in which charge carriers are generated and multiplied when exposed to light. They are widely used in high speed communication. APDs operate under reverse bias with a high peak electric field close to breakdown. Incident photons in the appropriate wavelength range, i.e., 300-1600 nm, create charge carriers (electrons and/or holes) in the semiconductor material. Charge carriers are accelerated toward opposite electrodes by the large reverse bias. The accelerated carriers then produce secondary carriers by impact ionization within the semiconductor material. The resultant avalanche can produce gains in excess of 10.sup.3. APDs can improve the sensitivity of optical receivers by more than 10 dB.
For long wavelength applications, e.g., wavelengths in the range 1000-1600 nm, the light absorption layer of the APD must be formed from a narrow band-gap semiconductor material. However, the large reverse bias typically creates excessive noise due to a large dark current flowing through the narrow band-gap material. To suppress this excessive noise, a separate layer of a material having a wider band-gap is provided, allowing avalanche multiplication to take place. An APD constructed in this manner is commonly known as a separate absorption and multiplication (SAM) APD.
Generally, there are two types of SAM-APD structure: planar structure or mesa structure. In mesa structure SAM-APDs, the multiplication layer is grown epitaxially. This provides precise control over the layer's thickness and impurity dopant concentration. Mesa structures, however, expose a high electric field region at the mesa surface. Until recently, passivation of this surface has not been adequately demonstrated, and mesa structure APDs have not been favored.
Properly-designed planar structure APDs have a lower electric field strength at the surface of the structure than mesa structure APDs. In planar structure APDs, the P-N junction is commonly formed by diffusing p-type dopants into epitaxially-grown n-type layers. The thickness of the multiplication layer is defined by the position of the P-N junction. FIG. 1 shows a conventional planar SAM-APD in which the n-type InP buffer layer 10, the n.sup.- InGaAs light absorption layer 11, the n InGaAsP intermediate layer 12, the n InP avalanche multiplication layer 13, and the n.sup.- InP window layer 14 are epitaxially grown in sequence on the n.sup.+ InP substrate 15. The p.sup.+ InP diffusion layer 16 and the p-type guard ring 17 are formed in the window layer 14 by selective diffusion or ion implantation techniques. The P-side electrode 18 is provided on the upper surface of the device, and the N-side electrode 19 is formed on the lower surface of the substrate 15.
In the SAM-APD just described, holes generated by light absorption in the n.sup.- InGaAs light absorption layer 11 drift into the n InP multiplication layer 13 where they initiate avalanche multiplication. Ideally, the APD is designed so that the field in the light absorption layer 11 is kept low enough to suppress the dark current. Holes generated in the n.sup.- InGaAs light absorption layer 11 are accumulated in the valence band of the hetereojunction formed between the light absorption layer 11 and the n-type InP multiplication layer 13. This increases the response time of the APD. To overcome this disadvantage, the n.sup.- InGaAsP intermediate layer 12 is disposed between the n.sup.- InGaAs light absorption layer 11 and the n-type InP multiplication layer 13.
To obtain high sensitivity, it is necessary to obtain uniform avalanche multiplication along the P-N junction 21 between the n.sup.- InP window layer 14 and the p.sup.+ InP layer 16. To that end, it is necessary to restrict the region of breakdown to the central portion of the APD, coextensive to a planar portion of the P-N junction 21. It has long been recognized that curvature increases the electric field strength. Therefore, for a given potential difference across the P-N junction 21, the electric field strength is greater in the curved portion 20 of the P-N junction than in the planar portion. This increased field strength can lead to premature breakdown at the curved portion 20. This breakdown is commonly known as edge breakdown.
To avoid edge breakdown, the guard ring 17 is provided to surround the p.sup.+ InP layer 16. The guard ring 17 is formed so that it creates a second P-N junction 22 between both the window layer 14 and the multiplication layer 13. The second P-N junction is generally deeper than the P-N junction 21. The guard ring effectively eliminates the curved portion 20 of the P-N junction 21.
In the conventional planar APD, described above, the n.sup.- InP window layer 14 is often given a low carrier concentration and is epitaxially grown on the n-type InP avalanche multiplication layer 13, which has a higher carrier concentration. P-N junction 22 is formed by selective diffusion, or implantation and annealing, at high temperatures of Be ions or the like into the window layer 14. P-N junction 21 is typically formed by selective diffusion of the p-type dopants forming the layer 16 into the layer 14 using Cd or Zn as a diffusion source.
To achieve an APD with high gain-bandwidth product, the P-N junction 21 is positioned as deep as possible so that it is located near or within the avalanche multiplication layer 13. Further, to obtain a short response time, the multiplication layer must have a high concentration of dopants. This requires a high degree of control of both the doping and thickness of the multiplication layer 13 and the guard ring 17 so that a sufficient quantity of photo-generated carriers are extracted to achieve the desired gain. Also, the electric field in the absorption layer 11 must be kept low to avoid excessive dark current.
The limitations of diffusion techniques for manufacturing APDs are manifest. The precision of the structures that can be made using diffusion is limited. For example, to achieve a gain-bandwidth product of 100 GHz, a uniformly-doped multiplication layer must have a thickness of about 0.7 .mu.m with a required precision of .+-.0.04 .mu.m. It is very difficult to achieve this degree of precision with diffusion technology. For similar reasons, it is difficult to control the doping and position of the guard ring accurately. This often leads to low fabrication yields and increased costs in the production of APDs. Low fabrication yield is a significant drawback in manufacturing an APD with high gain-bandwidth product.
In U.S. patent application Ser. No. 08/389,375, the inventors of the present invention disclosed a mesa-structure SAM-APD in which the problem of passivating the planar P-N junction at the mesa surface was solved by epitaxially regrowing a guard ring around the mesa. A cross-sectional view of the device is shown in FIG. 2. The n.sup.- InGaAs light absorption layer 23, the n.sup.- InGaAsP intermediate layer 24, the n InP avalanche multiplication layer 25 and the homogeneous p.sup.+ InP cap layer 27 are epitaxially grown in sequence on the n.sup.+ InP substrate 26. The multiplication layer and the cap layer form the planar P-N junction 28. A portion extending completely through the thickness of the cap layer 28, and a portion extending part-way through the thickness of the multiplication layer 25 are removed to form a mesa structure.
The guard ring 29 is epitaxially regrown on the multiplication layer, surrounding the mesa, and forms the second P-N junction 34 at the interface between the guard ring and the multiplication layer 25. The guard ring protects the planar P-N junction 28 which, prior to the guard ring being regrown, is exposed at the surface of the mesa structure. The guard ring 29 is preferably formed from the same semiconductor material as the p.sup.+ cap layer 27, but has a lower concentration of impurities to reduce the electric field in the guard ring.
The second P-N junction 34 includes the planar portion 36 and the curved portion 37. The planar portion 36 is located adjacent and parallel to the multiplication layer 25. The curved portion 37 is positioned closer to the light absorption layer 23 than the first P-N junction 28. In addition, the guard ring 29 preferably has a lower impurity concentration than the multiplication layer 25. This structure substantially reduces the electric field proximate to the first P-N junction, reducing the likelihood of edge breakdown.
The mesa-structure APD shown in FIG. 2 lacks a window layer corresponding to the window layer 14 of the conventional planar-structure APD shown in FIG. 1. Omitting the window layer greatly reduces avalanche build-up time, which decreases the response time of the mesa-structure APD compared to the conventional planar-structure APD.
The P-side electrode 30 is placed in contact with the cap layer 27. The N-side electrode 31 is formed on the lower surface of the semiconductor substrate 26.
Test results on the above-described mesa-structure SAM-APDs indicate that some residual edge breakdown occurs. The inventors' analysis of the cause of such residual edge breakdown indicates that the gain of the APD is not uniform across the first P-N junction, but has peaks where this junction intersects the side of the mesa structure. The inventors' analysis indicates that the cause of the gain peaks is the acute-angled corner 32 that remains at the interface between the mesa structure and the epitaxially regrown guard ring. The sharply-angled portion of highly-doped p-type material located adjacent the lightly doped multiplication layer and guard ring causes a sharp increase in the electric field strength at the ends of the first P-N junction.
Additionally, production yields of the above-described mesa-structure SAM-APDs were less than expected. When multiple APDs were made on a common substrate and a single guard ring layer is epitaxially regrown over the entire surface of the wafer, a subsequent additional mesa etch is required to isolate the guard rings of the devices from one another. When the guard rings are defined by etching, the peripheries of the guard rings need to be passivated by a second regrowth or by a film of a suitable dielectric, such as silicon nitride, to prevent excessive surface leakage current. The additional processing reduces the yield of good APDs.
What is needed is a SAM-APD that does not suffer from edge breakdown at the interface of the planar P-N junction and the guard ring. What is also needed is a SAM-APD in which a common guard ring layer does not have to be etched to define the guard rings of the APDs made on the same substrate, and that does not require additional processing to passivate the periphery of the guard rings.